Control of particle size and PSDs

Many polymerizations are performed in heterogeneous media, so that the final polymer product is obtained as a particulate material. The characteristics of the final particle size distribution (PSD) of the product maybe of fundamental importance for many applications. 

For instance, in order to increase the polymer concentration of polymer latexes, multimodal and/or broad PSDs may be required [47]. It must also be stressed that the PSD and the polymerization rates are coupled in many heterogeneous polymerizations. This leads to the definition of the fifth classical control problem, which is the control of some features of the PSD of the final polymer material. 

Manipulation of PSDs is generally attained through modification of surfactant concentrations (mostly in emulsion polymerizations) [48,49], agitation speeds (mostly in suspension polymerizations) [50], and initial catalyst size distributions and reaction times (residence time distributions in continuous reactors, mostly in coordination polymerizations) [51]. Effects of agitation speeds and surfactant concentrations on the PSD of polymer particles produced in suspension and emulsion polymerizations are discussed in detail in Chapters 5 and 6, respectively. When the catalyst is fed into the reactor as a solid material, as in typical polyolefin reactions, then the residence times and the initial PSD of the catalyst particles are used to manipulate the PSD of the final polymer product. Similar strategies are used in seeded emulsion polymerizations, where an initial load of preformed particles can be used to improve the control over the concentration of polymer particles in the latex and over the PSD of the final polymer product. 

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